Scientists have determined a close correlation between tumor cell surface receptors and metastasis. Metastasis is the transfer of neoplastic disease from one organ or part of an organ to another organ, the two organs not being directly connected. For example, the transfer of a melanoma from one tissue to the lung or liver. It has been determined that the process of metastasis is initiated by the detachment of tumor cells from the primary growth situs and is followed by the invasion by these detached tumor cells in surrounding tissues and blood vessels. The tumor cells disseminate in the blood circulation to distant organs where the tumor cells arrest and proliferate to form new tumor colonies. (1).
Cell surface constituents, such as glycoproteins, have been implicated in the mediation of cellular interactions related to recognition, adhesion, growth, differentiation and morphogenesis. (2) Cell adhesiveness which plays a roll in normal morphogenesis and homeostasis is also an important factor in the pathogenesis of cancer (3). For example, the copending European Patent Application PCT/EP 85/00557 to Max-Planck-Gesellschaft Zur Fo-rderung Der Wissenschaften E.V. discloses specific carbohydrate binding proteins of mammalian tumor cells which specifically recognize and bind to carbohydrate molecules on the cell surface of mammalian tumor cells. These lectins have been used for diagnostic purposes to detect mammalian tumor cells and are proposed as a pharmaceutical composition for treating malignant neo-plasms by inhibiting metastasation.
It has been shown that intercellular recognition and adhesion among tumor cells and host cells lead to the formation of tumor cell emboli, the formation of the emboli being correlated to the incidence of metastasis (4). Accordingly, intercellular recognition receptors on cell surfaces bound by an antigenic component should inhibit the formation of tumor cell emboli thereby inhibiting metastasis.
For example, when B16-F1 melanoma cells were cultured as spheroids on a non-adhesive substrate (5), the cells showed an increased propensity to colonize in the lungs of C57BL/6 mice in a reversible manner. This was found to be correlated to the increased expression of a cell surface sialylated Peanut Agglutinin (PNA) binding glycoprotein, gp78 (6). Pretreatment of the cells with polyclonal anti-gp78 antibodies prior to intravenous injection in sygenic mice induced an effect similar to that of altered cell shape growth conditions, the treated cells exhibiting a two fold increase in lung colonization. This result suggested a role for the gp78 molecule in metastasis (7).
As stated above, the molecular nature of the cell surface proteins, including certain carbohydrate-binding-proteins (CBP) mediate such cognitive cellular interactions, and their role as tumor surface lectins in anchorage independent growth in vitro and in tumor embolization and heterotypic aggregation during the metastatic process in vivo has been suggested (8-10). For example, the presence of lactose binding lectin in various murine and human tumor cells has been described (11-13). Quantitative analysis of tumor cell arrest and survival has shown that most of the intravenously inoculated tumor cells are initially trapped in the first capilliary bed encountered by the cells, that being lung tissue (14). Complimenting this finding is the finding that isolated tumor cells rarely lead to hematogenous metastasis of tumors (15). Further tumor emboli are predominantly responsible for tumor dissemination and arrest at secondary sites (16). The importance of tumor cell aggregation has been further demonstrated by the correlation established between the propensity of tumor cells to undergo homotypic aggregation or to participate in heterotypic aggregation in vitro and their metastatic potential in vivo (17). It is further assumed that the specific arrest of tumor emboli in particular organs requires cognitive interactions between the tumor cells and post capilliary endothelial cells of the basement membrane (18, 19).
Researchers have reported on the involvement of carbohydrate residues in the mediation of adhesion between cells and the ability of simple sugars, glycopeptides and glycoproteins to inhibit the aggregation of certain types of cells (20). In other words, simple sugars have been shown to prevent cell to cell aggregation. Another report shows that fucosyltransferase from human milk immobilized on polystyrene plates enhances the adhesion of cells by binding cell surface oligosaccharide acceptors. This adhesion can be inhibited by oligosaccharides containing the sugar sequence galactosyl (beta-1-4)N-Acetylglucosamine (21).
With regard to function of glycolipids on the cell surface, studies on the adhesion of liposomes containing various glycolipids to Hella cell carcinomas as a model of Hella cell membrane function demonstrated, that the glycolipids containing terminal galactose residues were the most active in promoting adhesion (22), and the fundamental role of a 34 KD, a galactoside binding lactin 34,000 in cell transformation and metastasis has been demonstrated (11-13).
Most cell types contain lectins of different molecular weights and different sugar specificities including lactosides, mannose and fucose. In general, different types of lectins have different cellular compartments (23).
Finally, the absorption of the Agrobacterium Tumefaciens strain B6 was increased if citrus pectin, polygalacturonic acid or demethylated pectin was included in the inoculum (24). In another study it was shown that the administration of citrus pectin in vivo resulted in a high incidence of colorectal tumors (25).
The present invention provides a modified citrus pectin able to affect motility of malignant tumor cells and prevent lung colonization in vivo.